This invention relates to improved gypsum products. More specifically, it relates to an improved gypsum slurry that is flowable at low water concentrations, with less expense and less retardive effects than using carboxylate dispersants alone.
Gypsum products are commonly used as building materials for many reasons, such as wallboard. Wallboard sheets are easily joined together to make continuous walls of any size and shape. They are easily patched and have fire and sound proofing properties. Decorative finishes, such as wallpaper or paint readily adhere to plaster or wallboard surfaces to allow for a large variety of decorating options.
The strength of gypsum products made from full density slurries is inversely proportional to the amount of water used in their manufacture. Some of the water that is added to the gypsum slurry is used to hydrate the calcined gypsum, also known as calcium sulfate hemihydrate, to form an interlocking matrix of calcium sulfate dihydrate crystals. Excess water evaporates or is driven off in a kiln, leaving voids in the matrix once occupied by the water. Where large amounts of water were used to fluidize the gypsum slurry, more and larger voids remain in the product when it is completely dry. These voids decrease the product density and strength in the finished product.
Attempts have been made to reduce the amount of water used to make a fluid slurry using dispersants. Polycarboxylate superplasticizers are very effective in allowing water reduction and the resultant increase in product strength, however, there are disadvantages known to be associated with use of large doses of polycarboxylate dispersants. These materials are relatively expensive. When used in large doses, polycarboxylate dispersants can be one of the single most expensive additives in making gypsum products. The high price of this component can overcome the narrow margins afforded these products in a highly competitive marketplace.
Another disadvantage associated with polycarboxylate dispersants is the retardation of the setting reaction. Gypsum board is made on high speed production lines where the slurry is mixed, poured, shaped and dried in a matter of minutes. The board must be able to hold its shape to be moved from one conveyor line to another to put the board into the kiln. Damage can occur if the boards have not attained a minimum green strength by the time they are stacked and wrapped for shipping. If the board line has to be slowed down because the board is not sufficiently set to move on to the next step in the process, production costs are driven up, resulting in an economically uncompetitive product.
Lime has been used in plaster to improve its workability. It gives the plaster a good “feel”, imparting a smoothness and plasticity that makes it easy to trowel. Since it is alkaline, lime acts to make some retarders more efficient, increasing the open time of the plaster. Finally, the lime present in the plaster oxidizes over time to form calcium carbonate which gives the surface a hardness beyond that obtainable with plaster alone.
U.S. Pat. No. 5,718,759 teaches the addition of silicates to mixtures of beta-calcined gypsum and cement. In the examples, lignosulfates or naphthalene sulfonates are used as water-reducing agents. The addition of pozzolanic materials, including silicates, is credited with reducing expansion due to the formation of ettringite. The composition is suggested for use in building materials, such as backer boards, floor underlayments, road patching materials fire-stopping materials and fiberboard.
Luongo, in U.S. Pat. No. 6,391,958, teaches a novel wallboard composition combining gypsum with sodium silicates and a synthetic, cross-linking binder. Vinyl acetate polymers were the preferred cross-linking binder. The addition of sodium silicates reduces the amount of calcined gypsum that is needed to make a given number of panels. The weight of the building panel is reduced, making it easier for workers to move the panels before and during installations.
A number of polycarboxylate dispersants are disclosed in U.S. Pat. No. 6,005,040. In one embodiment of the invention, a water soluble polymer is disclosed having one repeating unit of the formula:

where P is an integer from 1-10 and R2, R3, R4, R5 and R6 are not all hydrogens, but any one of them can be a hydrogen. The polymer also includes a water-soluble repeating unit selected from a group containing acrylic acid and methacrylic acid, among others. The polymer is used in an unfired, ceramic precursor material. Other polymers are disclosed in the application, including some for use in the core forming process of gypsum wallboard or the preparation of gypsum slurries.
The prior art has failed to adequately address the problem of improving the efficacy of a given polycarboxylate dispersant. Improving the efficacy of a dispersant would reduce the cost of the dispersant while maintaining the reasonable price of gypsum products.
Thus, there is a need in the art to reduce the dosage of dispersants used in a gypsum slurry while maintaining flowability of the slurry. Reduction in dispersant use would result in saving of costs spent on the dispersant and would reduce adverse side effects, such as set retardation.